1. Field of the Invention
This invention relates to friction elements for railroad brake shoes and more particularly to a friction element formed by combining materials having characteristics which may be handled conveniently and safely without imposing a significant threat to human health and a safe environment.
2. Prior Art
A typical composition friction element usable in a railroad brake shoe is disclosed in U.S. Pat. No. 3,227,249, a joint invention of the inventor herein. The components of the element include an organic bond matrix, a mineral filler and cryolite, i.e. sodium-aluminum fluoride. The bond is made of synthetic rubber and resin which are curable under temperature and pressure. The hard mineral fillers suggested are cast iron turnings, white iron grit and calcined kyanite, i.e. aluminum silicate. Additional inorganic fillers such as litharge and lead powders are used to stiffen the bond and act as a lubricant while asbestos fiber is added to improve structural strength.
A further composition friction element is taught by U.S. Pat. No. 3,885,066. This element comprises 15-35% by weight of a resin binder, 45-65% by weight asbestos, and 3-10% by weight of one or more fillers. The fillers impart increased hardness and wear resistance to the brake shoe and function as friction modifiers. Fillers adding to the hardness of the composite include barytes, i.e. barium sulfate, alumina, i.e. aluminum oxide, zinc and limestone, i.e. calcium carbonate. Friction modifying fillers include brass powder, iron powder, carbon black, ground cork and aldehyde condensation products of cashew nut liquid.
A still further composite friction element is disclosed in U.S. Pat. No. 3,959,194 wherein the element is particularly formulated for use with relatively soft gunite iron railroad car wheels. The friction element is composed of 3-25% by weight of a rubber binder, 20-70% by weight of an inorganic filler and 2-12% by weight of fiber. The fiber component may be asbestos or a cellulose based material such as wood, sisal, jute and rayon fibers. The rubber binder may be made from a natural or synthetic rubber or other elastomeric material which is vulcanized or otherwise cured to form a hard material in which the other components are distributed. A phenolic resin at a concentration of 1-30% by weight is used as a strengthening or stiffening agent for the rubber matrix. Phenolic resins suggested include oil-modified two-stage powdered phenol formaldehyde resin and a liquid resin prepared from natural sources of phenol derivatives such as aldehyde reacted cashew nut shell oil. Hexamethylene-tetramine may be used as a curing agent. Graphite, cast iron, calcium carbonate, barytes and carbon black are suggested as inorganic fillers.
A most recent development in friction composites is disclosed in U.S. Pat. No. 4,137,214. This composite is particularly useful for automotive use. The composition is asbestos-free and includes a thermosetting resin and cashew particles. As a substitute for asbestos, fiberglas, mineral wool, silica fibers, carbon fibers, boron fibers, cotton, rayon, polyamide or polyester fibers as well as tungsten and steel fibers are suggested. Lastly, the friction composite includes powdered inorganic compounds such as zinc oxide and calcium carbonate each having a Moh's hardness rating greater than 2.0 and less than 5.0.
Other composite friction element developments are disclosed in U.S. Pat. No. 4,219,452. By the invention disclosed therein an aramid fiber was successfully substituted for asbestos.
Before a composite brake shoe can be sold for use by the United States railroad industry, it must satisfy certain minimum standards. These standards are met when a brake shoe successfully passes certain tests that have been established by the Association of American Railroads. Four such tests are used and include fire screen burn test, a series of drag tests, a series of stop tests and a static coefficient of friction test. In each case, the tested brake shoe selectively engages a railroad car wheel driven by a dynamometer. Although not a formal part of the AAR testing procedure, it is desireable that a composite friction element have low squeal characteristics, inhibit excessive metal pickup from the wheel tread, and be spark resistant.